This invention relates to apparatus for examining the human body with penetrating radiation such as X-ray and gamma radiation. The term X-ray will be used herein to embrace both types of radiation for the sake of brevity. The new apparatus is especially applicable to obtaining X-ray images of layers in a human body by means of computed tomography.
Background information on computed tomography may be obtained from an article entitled "Image Reconstruction From Projections" by R. Gordon, G. T. Herman and S. A. Johnson in Scientific American, Oct. 1975, Vol. 233, No. 4, p. 56. Additional background material is contained in U.S. Pat. Nos. 3,881,110 to Hounsfield et al and 3,867,634 to Hounsfield. Additionally, U.S. Pat. Nos. 4,010,370 and 4,010,371, both dated Mar. 1, 1977, show computerized tomography scanners in which the X-ray source or detector is shifted for various purposes.
The present invention is an improvement in the general type of X-ray scanning apparatus or computed tomography apparatus which is illustrated in the pending application of Kelman et al for a "Gantry for Computed Tomography", Ser. No. 771,863, filed Feb. 25, 1977. The invention may also be used in the type of X-ray scanning or computed tomography apparatus shown in the pending application of Redington et al for an "X-ray Body Scanner for Computerized Tomography", Ser. No. 723,799, filed Sept. 16, 1976. Both applications are assigned to the assignee of the present application. A typical X-ray detector array which may be used in the apparatus that will be hereinafter described may be seen in the pending application of H. R. Cummings for a "Detector Assembly for Body Scanner", Ser. No. 727,260, filed Sept. 27, 1976.
In one arrangement for performing computed tomography, a patient is supported for being translated along a longitudinal axis which is usually horizontally disposed. The axis coincides with the center of rotation of a rotatable base which has an X-ray source on one side of its center of rotation and a multiple array of X-ray detectors on the other side. A fan-shaped X-ray beam, that is thin in the longitudinal direction, is projected through the patient as the base rotates so that the detectors may develop signals indicative of X-ray transmission characteristics along a plurality of paths through a subject undergoing examination. Analog signals representative of X-ray attenuation by all of the volume elements in a layer of the body at various rotational angles are then converted to digital signals which are used by a computer to produce signals which are used for controlling a cathode ray tube to display a reconstructed image of the layer. A collimator is coupled with the X-ray source which forms the beam into a fan-shape having a layer thickness of typically about 1 cm. The boundary rays have a suitable angle of divergence for spreading over the entire length of the detector but, preferably, there should be no overlapping of the ends of the detector array. It will be evident that in such an arrangement the beam must be sufficiently divergent at the plane in which the patient is located for all parts across a patient to be within the beam and to project an image which is apportioned to the respective cells in the detector array. Algorithms to which the computer is programmed for reconstructing the image of a body layer from the digital signals produce more accurate and consistent results if all cells in the detector intercept a part of the attenuated X-ray beam. Anomalies may be produced if some of the cells receive a direct unattenuated portion of the X-ray beam or if they receive no x-radiation. It is, therefore, desirable for the scanning X-ray beam to cover the entire field of the detector at all times.